Vortex flow measurement devices are often used for measuring flow of fluids flowing in pipes, and especially is this the case for high temperature and/or high pressure gas- or vapor-flows. Typical variants in construction and use of such vortex flow measurement devices are described, for example, in the following U.S. Pat. Nos. 4,448,081, 4,523,477, 4,716,770, 4,807,481, 4,876,897, 4,973,062, 5,060,522, 5,121,658, 5,152,181, 5,321,990, 5,429,001, 5,569,859, 5,804,740, 6,003,384, 6,170,338, 6,351,999, and 6,352,000.
The manner in which usual vortex flow measurement devices function is based on using periodic pressure fluctuations in a Karman vortex street. These arise, as is known, when a fluid is allowed to flow against an obstacle to the flow, for instance against a bluff body. Vortices are periodically released from this bluff body to its downstream side and these form the mentioned vortex street. The repetition frequency, with which the vortices are formed, is proportional over an extended Reynolds number range to the flow velocity of the fluid, which means that volume flow, i.e. volume flow rate, can be measured practically directly by means of such vortex flow measurement devices.
A vortex flow measurement device of the described kind usually includes a measurement tube of preselected length, in whose lumen the mentioned bluff body is arranged, preferably along a diameter of the measurement tube. The inlet and outlet ends of the measurement tube are connected to the pipe containing the fluid to be measured, so that, during operation of the vortex flow measurement device, the fluid can be allowed to flow through the measurement tube and, consequently, be made to flow against the bluff body.
Characteristic for such vortex-producing bluff bodies is that they exhibit on their upstream side an essentially flat, flow impingement surface, which laterally abruptly ends, in order to form at least two, as sharp as possible, separation edges. Beginning at the separation edges, the bluff bodies then narrow on the downstream side. This can occur e.g. continuously, or, as shown e.g. in U.S. Pat. No. 5,569,859, even stepwise. Besides the two separation edges, the bluff body can also exhibit other separation edges.
Finally, at least one sensor element is situated in the bluff body or is arranged downstream from the bluff body internally at the wall of the measurement tube or externally at the wall or within the wall. The pressure fluctuations associated with the vortices are registered and changed into electrically processable signals by means of the e.g. capacitively, inductively or piezoelectrically operating sensor element arranged in the bluff body itself or downstream therefrom. It can also be an ultrasonic sensor. The electrically processable signals have a frequency which is directly proportional to the volume flow rate in the measurement tube.
The electrical signals produced by the sensor element are processed by a corresponding evaluation electronics of the vortex flow measurement device and can e.g. be displayed on location and/or be further processed in higher level evaluation units.
As already indicated, a flow velocity of the fluid to be measured, and/or the volume flow rate derived therefrom, can be practically directly measured by means of such vortex flow measurement devices. Starting with the measured volume flow rate and an instantaneous fluid density registered simultaneously therewith, or even subsequently, an instantaneous mass flow rate can be indirectly determined, as e.g. also described in WO-A 95/11,425, and the following U.S. Pat. Nos. 4,876,897, 4,941,361, 5,121,658, and 5,429,001. As shown in the U.S. Pat. Nos. 4,448,081, 4,523,477, 4,807,481, 4,973,062, 5,060,522, 5,152,181, 5,429,001, 5,804,740, and 6,170,338, it is furthermore also possible to determine the mass flow rate by means of such vortex flow measurement devices using the measured volume flow rate and a dynamic pressure acting in the fluid in the flow direction.
Especially in WO-A 95/11,425, and the U.S. Pat. Nos. 5,429,001, and 6,170,338, it is proposed to determine the dynamic pressure using an amplitude curve, particularly a time-averaged amplitude curve, of a vortex measurement signal changing periodically with the repetition frequency of the vortex, which signal corresponds to a plot of pressure versus time as registered locally in the vortex street. Investigations have, however, shown that the amplitude curve or also the average amplitude curve of such a vortex measurement signal is proportional to the dynamic pressure practically only in the case of a steady state flow. Beyond this, it is proposed e.g. in U.S. Pat. No. 4,448,081 to determine the dynamic pressure on the basis of amplitude versus time of elastic deformations of the bluff body impinged by the flow.